WO2002003541A2

WO2002003541A2 - Digitally modulated rf amplifier system having reduced intermodulation energy at rf channel edges

Info

Publication number: WO2002003541A2
Application number: PCT/US2001/020943
Authority: WO
Inventors: David Sparano
Original assignee: Harris Corporation
Priority date: 2000-06-30
Filing date: 2001-06-30
Publication date: 2002-01-10
Also published as: AU2001271725A1; WO2002003541A3

Abstract

A digitally modulated RF amplifier system having improved adjacent sideband distortion reduction at spaced apart frequencies located essentially at the edges of a desired frequency channel. The system includes a digital modulator that receives a digital information signal and modulates a received RF carrier signal with the digital information signal to provide a digitally modulated RF signal within a first RF channel having a bandwidth of N and which is interposed between and is adjacent to second and third channels each having bandwidth N. A non-linear power amplifier has an input that receives and amplifies the modulated RF signal and exhibits the characteristic of normally providing unwanted intermodulation energy at frequencies at the edges of the first RF channel and that extend into the adjacent second and third frequency channels. A transmission line matcher is interposed between the modulator and the power amplifier to optomize the inpurt return loss of the power amplifier. The transmission line has a length chosen to simulaneously maximize the return loss at first and second frequencies located essentially at the edges of the first frequency channel to reduce said unwanted intermodulation energy simultaneously at the edges.

Description

DIGITALLY MODULATED RF AMPLIFIER SYSTEM HAVING REDUCED INTERMODULATION ENERGY AT RF CHANNEL EDGES

The present invention relates to RF communication systems and is particularly directed to a digitally modulated RF amplifier system having reduced intermodulation energy at the RF channel edges to minimize interference with adjacent channel signals.

In the current state of the art, amplifiers are used to provide power amplification of RF signals for use in radio transmission of digitally modulated commtinication signals. Frequently, these power amplifiers (PA) are operated at the nonlinear region of their operating curve. The result is the nonlinear amplification of the desired signal and the creation of spurious intermodulation energy that falls into adjacent channels, thereby causing interference. Often, a transmission line matcher is used before the PA input to maximize the PA stage input return loss. That is, to maximize the efficient transfer of RF energy from the preceding driver stage into the power amplifier. This transmission line matcher is typically adjusted to maximize the PA input return loss at the center of the desired channel and often times over the widest range of frequencies within the channel as possible.^' The frequency width versus quality of match is 'fundamentally limited by the Bode-Fano criteria. As a result, it is not possible to simultaneously maximize the PA stage input return loss at more than one frequency in, or immediately adjacent to, the desired channel. At the PA stage output, a channel bandpass filter removes the intermodulation energy at frequencies greater than a certato nominal range from the desired channel edges. The channel bandpass filter is not able to significantly remove the intermodulation energy right at the desired channel edges, because to do so would create excessive group delay distortion, thereby corrupting the desired in-channel signal. The present invention includes a digitally modulated RF amplifier system having improved adjacent sideband distortion reduction at spaced apart frequencies located essentially at the edges of a desired frequency channel and comprising a digital modulator that receives a digital information signal and modulates a received RF carrier signal with said digital information signal to provide a digitally modulated RF signal within a first RF channel having a bandwidth of N and which is interposed between and adjacent to second and third channels each having a bandwidth N, a non-linear power amplifier having an input that receives and amplifies said modulated RF signal and exhibits the characteristic of normally providing unwanted intermodulation energy at frequencies at the edges of said first RF channel and that extend into said adjacent second and third frequency channels, a transmission line matcher interposed in between said modulator and said power amplifier to optimize the input return loss of said power amplifier, an elongated RF transmission line interposed between said transmission line matcher and the input of said power amplifier, said transmission line having a length chosen to simultaneously maximize the return loss at first and second frequencies located essentially at the edges of said first frequency channel to thereby reduce said unwanted intermodulation energy simultaneously at said edges, and, an output bandpass filter to remove intermodulation energy at frequencies beyond said channel edges.

Conveniently, the invention is directed toward improvements in reducing adjacent sideband distortion in a digitally modulated RF amplifier system. The improvement contemplates reducing intermodulation energy at the edges of a desired frequency channel so as to minimize interference with adjacent frequency channels.

In accordance with the present invention, a digitally modulated RF amplifier system is provided having improved adjacent sideband distortion reduction at spaced apart frequencies located essentially at the edges of a desired frequency channel. The system employs a digital modulator that receives a digital information signal and modulates a received RF carrier signal with the digital information signal to provide a digitally modulated RF signal within a first RF channel having a bandwidth of N and which is interposed between and adjacent to second and third channels each having abandwidthN. Anon-linear power amplifier receives and amplifies the modulated RF signal. This amplifier exhibits the characteristic of normally providing unwanted intermodulation energy at frequencies located at the edges of the first RF channel and which extend into the adjacent second and third frequency channels. A transmission line matcher is interposed between the modulator and the power amplifier. An elongated RF transmission line is interposed between the transmission line matcher and the input of the power amplifier. The transmission line is a length chosen to maximize the return loss at first and second frequencies located essentially at the edges of said fi st frequency channel to thereby reduce the unwanted intermodulation energy at those edges. An output bandpass filter removes intermodulation energy at frequencies beyond the channel edges.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:

Fig. 1 is a block diagram illustration of a typical prior art RF amplifier system; Fig. 2 is a block diagram illustration of a digitally modulated RF amplifier system Fig. 3 is a graphical illustration of input return loss with respect to frequency which is helpful in explaining the invention herein;

Fig.4 is a graphical illustration of output amplitude with respect to frequency and which is helpful in explaining the operation of the invention herein;

Fig.5 is graphical illustration of output amplitude with respect to frequency and which is helpful in describing the invention herein;

Fig.6 is a graphical illustration of output amplitude with respect to frequency and which is helpful in describing the invention herein;

Fig. 7 is a schematic-block diagram illustration in greater detail than that of Fig.2; and,

Fig. 8 is a schematic illustration of a second embocliment of the invention.

Fig.1 illustrates a typical RF amplifier. In this example, an input information signal from a source 10 is employed for the purposes of modulating an RF carrier signal from RF carrier generator 12 at a signal modulator 14. The modulated RF signal is then applied to an intermediate power amplifier 16. The output from the intermediate power amplifier 16 may be sampled with a reverse coupler 18 and supplied to a spectrum analyzer 20 for viewing the input return loss from a downstream non-linear power amplifier 22. An adjustable transmission line matcher 24 is interposed between the intermediate power amplifier and the non-linear power amplifier 22. The transmission line matcher is used to maximize the power amplifier input return loss. This maximizes the efficient transfer of RF energy from the preceding driver stage in the form of the intermediate power amplifier 16 into the power amplifier 22. The output from the non-linear power amplifier 22 is supplied to a bandpass filter 23 which removes the intermodulation energy at frequencies greater than a certain nominal range prior to applying the signal to an antenna 26 for broadcasting.

In the prior art discussed with reference to Fig. 1, the non-linear power amplifier 22 is frequently a tuned amplifier such as an inductive output tube (IOT). It may also be a solid state circuit including a transistor or a field effect transistor. In any case, the input to such electronic amplifying device includes an input tuned circuit. Frequently, these power amplifiers are operated in a non-linear region of their operating curve. The result is non-linear amplification of the information signal and the creation of spurious intermodulation energy that falls into adjacent frequency channels, thereby causing interference. In digital television (DTV), the channels extend for 6 MHz. For example, channel 17 extends from 488 MHz to 494 MHz. The adjacent channels are channel 16 and 18. Channel 16 extends from 482 MHz to 488 MHz, whereas channel 18 extends from 494 MHz to 500 MHz. Fig. 3 illustrates input return loss versus frequency and Fig. 4 which illustrates output amplitude versus frequency. The desired channel 17 is illustrated as being interposed between channels 16 and 18. Each channel is 6 MHz wide as is indicated for channel 17. As noted above, non-linear amplification of the information signal by the power amplifier 22 results in creation of spurious intermodulation energy that falls into adjacent channels and this is indicated in Fig.4 by the dotted curve 40 on either side of channel 17 and spreading into channels 16 and 18. This spurious energy in the adjacent channels causes interference.

Frequently, the tiansmissionline matcher 24 is adjusted to rriaximize the efficient transfer of RF energy to the power amplifier 22. Generally, this matcher 24 is adjusted to maximize the power amplifier input return loss at the center frequency F_c at the center of the desired channel, in this case channel 17. Sometimes the matcher is adjusted to maximize the input return loss over the widest range of frequencies within the desired channel as is possible. However, the depth of the maximum input return loss is not as great as that optimized for the center frequency F_c as is shown in Fig. 3. The frequency width versus quality of match is fundamentally limited by the Bode-Fano criteria. As a result it is not possible to simultaneously maximize the power amplifier input return loss of more than one frequency in, or immediately adjacent to, the desired channel.

The channel bandpass filter 23 located in the output of the power amplifier 22 removes the intermodulation energy at frequencies greater than a the desired edges as indicated by the mask curve 50 in Fig. 5. However, the bandpass filter 23 does not significantly remove the intermodulation energy right at the desired channel edges because to do so would create excessive group delay distortion, thereby corrupting the desired in-channel signal.

The present invention is directed toward maximizing the return loss simultaneously at first and second frequencies which are located essentially at the edges of the desired frequency channel. In the example being illustrated in Fig. 3 this is channel 17 having a bandwidth of 6 MHz. Maximizing the return loss at two frequencies right at the edges of the desired frequency channels drastically reduces the unwanted intermodulation energy simultaneously at the channel edges. As the signal is being transmitted by the antenna 26, the output amplitude of the intermodulation energy the edges of desired channel is substantially reduced as is seen at curve nulls 62 in Fig.4. The output filter 23 removes the intermodulation energy at frequencies just beyond the channel edges, as is seen by the outline of the mask 50 in Fig. 5.

Fig. 6 illustrates actual test results providing an output signal similar to that as shown in Fig. 5. In this example, the curve 70 has nulls 72 and 74 at first and second channel edges. These nulls show a reduction from the peak level of approximately -36.9 dB.

The invention is illustrated in Fig.2 wherein components which correspond with those in Fig. 1 are identified with like character references and only the addition of the elongated transmission line 60 is presented between the transmission line matcher 24 and the non-linear power amplifier 22.

Preferably the non-linear amplifier 22 is a tuned amplifier such as an inductive output tube (IOT). Such amplifiers have an input tuned circuit sometimes referred to as a resonant input or cavity. Fig. 7 illustrates an IOT non-linear power amplifier 22A of the grounded grid type. The input tuned circuit (which normally takes the form of an annular cavity) is illustrated electrically herein as tuned circuit 80 interposed between ground and the cathode of IOT 22A. This tuned circuit has an adjustable inductor 82 and an adjustable capacitor 84. The elongated transmission line 60 is illustrated as being a coaxial cable having its outer conductor connected to ground and its inner conductor connected through to an inductor 86 to ground. The adjustable transmission line matcher 24 is illustrated herein as including a pair of adjustable plates 24A and 24B which are slidable along the associated conductor. A reverse coupler 18 obtains a sample of the input return loss for application to a spectrum analyzer or the like. The IOT 22A is illustrated as having an output circuit including the filter 23. The coupler 25 is located between the filter 23 and the antenna 26 for providing a sample of the output signal and this may be applied to a spectrum analyzer so that an output as shown in Fig. 6 may be obtained.

As previously pointed out, an additional length of RF transmission line 60 is added between the transmission line matcher 24 and the input circuit 80 associated with the power amplifier 22 A. The electrical length from the matcher 24 to the input tuned circuit 80 is adjusted so as to be 180 degrees (λ/2) at a frequency equivalent to that of the desired signal bandwidth (i.e., the matcher-cable-power amplifier input is equal to 180 degrees at 6 MHz for US DTV television signals). The elongated transmission line 60 modifies the electrical performance of the matcher-power amplifier input combination such that the return loss into the power amplifier is simultaneously optimized at both edges of the desired channel at frequencies F_x and F, as discussed hereinbefore with reference to Fig.3. Depending upon the type of transmission line cable employed, the additional cable length can be tens of feet for US DTV television channels and hundreds of feet for more narrow band digital signals. For example, the electrical length of the transmission line matcher may be on the order of 1 foot. The IOT may be provided with input coaxial cable of a length of approximately 7 feet. This, however, will not provide the optimized return losses at the edges of the desired channel. To achieve this, additional length of transmission line 60 is added. This, for example, may be on the order of 23 feet in order to achieve the matcher-cable-power amplifier input of 180 degrees at 6 MHz.

Fig. 8 is similar to that of Fig. 7 and like components are identified with like character references. The IOT 22A of Fig.7 is replaced in Fig.8 with an NPN transistor 22B having its base connected to ground.

The present invention is based on the principal of maximizing negative feedback in tuned power amplifiers of the grounded grid, ground base, or grounded gate variety. In such amplifiers, the impedance of the input parallel resonant circuit across the cathode and control element (i.e., grid, base or gate), in addition to coupling energy into the amplifier, also serves as a negative feedback which minimizes the non-linear characteristics of the amplifying device. At those frequencies where the input resonant circuit impedance is maximized, the negative feedback is greatest, the amplifier linearity is improved, and undesired intermodulation energy is reduced at the power amplifier output. The input resonant circuit impedance presented to the amplifying device is maximized when the return loss into the tuned input circuit is maximized or, alternatively, when the input match is optimized. To simultaneously optimize the input resonant circuit match at both channel edges simultaneously, it is necessary to modify the transmission line matcher-power amplifier input circuit combination such that it be resonant at 6 MHz for US DTV channels. With such a configuration, the impedance matching and tuning properties of the transmission line matcher and power amplifier input circuit will repeat every 6 MHz in and around the RF channel frequency for the cited example. In this way it is possible to simultaneously optimize the match and thereby minimize intermodulation energy at any two frequencies separated by the bandwidth frequency (i.e., both edges of the desired channel bandwidth). A digitally modulated RF amplifier system having improved adjacent sideband distortion reduction at spaced apart frequencies located essentially at the edges of a desired frequency channel. The system includes a digital modulator that receives a digital information signal and modulates a received RF carrier signal with the digital information signal to provide a digitally modulated RF signal within a first RF channel having a bandwidth of N and which is interposed between and is adjacent to second and third channels each having a bandwidth N. A non-linear, power amplifier has an input that receives and amplifies the modulated RF signal and exhibits the characteristic of normally providing unwanted intermodulation energy at frequencies at the edges of the first RF channel and that extend into the adjacent second and third frequency channels. A transmission line matcher is interposed between the modulator and the power amplifier to optimize the input return loss of the power amplifier. An elongated RF transmission line is interposed between the transmission line matcher and the input of the power amplifier. The transmission line has a length chosen to- simultaneously maximize the return loss at first and second frequencies located essentially at the edges of the first frequency channel to reduce said unwanted intermodulation energy simultaneously at the edges.

Claims

Claims:

1. A digitally modulated RF amplifier system having improved adjacent sideband distortion reduction at spaced apart frequencies located essentially at the edges of a desired frequency channel and comprising a digital modulator that receives a digital information signal and modulates a received RF carrier signal with said digital information signal to provide a digitally modulated RF signal within a first RF channel having a bandwidth of N and which is interposed between and adjacent to second and third channels each having a bandwidth N, a non-linear power amplifier having an input that receives and amplifies said modulated RF signal and exhibits the characteristic of normally providing unwanted intermodulation energy at frequencies at the edges of said first RF channel and that extend into said adjacent second and third frequency channels, a transmission line matcher interposed in between said modulator and said power amplifier to optimize the input return loss of said power amplifier, an elongated RF transmission line interposed betweensaid tiansmissionline matcher and the input of said power amplifier, said transmission line having a length chosen to simultaneously maximize the return loss at first and second frequencies located essentially at the edges of said first frequency channel to thereby reduce said unwanted intermodulation energy simultaneously at said edges, and, an output bandpass filter to remove intermodulation energy at frequencies beyond said channel edges.

2. A system as claimed in claim 1 wherein said input of said amplifier is a tuned circuit, and in which said transmission line matcher, said elongated transmission line and said tuned circuit are connected together in series to define a series circuit.

3. A system as claimed in claim 2 wherein said elongated transmission line has a length chosen such that the electrical length of said series circuit is 180 degrees at a frequency equivalent to .that of said bandwidth N, and preferably said bandwidth N is 6 MHz.

4. A system as claimed in clai 1 wherein said amplifier includes an inductive output tube (IOT) having an input tuned circuit, and in which said transmission line matcher, said elongated transmission line and said tuned circuit are connected together in series to define a series circuit.

5. A system as claimed in claim 4 wherein said elongated transmission line has a length chosen such that the electrical length of said series circuit is 180 degrees at a frequency equivalent to that of said bandwidth N, and preferably said bandwidth N is 6 MHz.

6 A system- as claimed in claim 1 wherein said amplifier includes a transistor having an input tuned circuit.

7. A system as claimed in claim 11 wherein said transmission line matcher, said elongated transmission line and said tuned circuit are connected together in series to define a series circuit.